Graft polymers, formed between epoxy resins and addition polymers, generally formed from monomer including an acrylic acid, are suggested in the earlier applications referred to above, and in certain prior art for use in coating compositions.
In the art prior to these applications, it appears that the so-called graft polymer has been an adduct formed by an esterification reaction between an acidic addition polymer and an epoxy resin. The techniques for making these epoxy-based acidic adduct resins water-dispersible, through inversion with amines, and for cross-linking them with added aminoplast, are well known. None of the prior art adduct products has been fully satisfactory. The adducts and coating compositions formed from them do not have good resistance to weather, because the adducts are susceptible to hydrolysis.
In application Ser. No. 685,246, there are disclosed processes for making water-reducible curable resinous compositions and coatings. In one embodiment, the application discloses a process for preparing a curable resinous composition having an Acid Number of at least 30, by reacting together at 90.degree. C. to 130.degree. C. an aromatic diepoxide resin having a molecular weight above 1,000 and addition polymerizable monomer of which from 10% to 80% by weight is an acrylic acid, the diepoxide resin being present in sufficient quantity to provide from 30% to 90% by weight of the initial reaction mixture, in the presence of a free radical initiator of the benzoyl peroxide type. During the reaction there is simultaneous addition polymerization of the monomer through its ethylenic unsaturation and grafting of addition polymer to the diepoxide resin. The graft polymer resinous reaction product is characterized by the substantial absence of hydrolyzable functional groups. The acid functionality of the reaction mixture is sufficiently high to effect stable dispersion of the product in a basic aqueous medium.
As is more particularly pointed out in patent application Ser. No. 788,611, now U.S. Pat. No. 4,212,781 the resinous reaction product produced contains three polymeric components, namely, the graft polymer ungrafted diepoxide resin, and ungrafted addition polymer.
As is pointed out in Ser. No. 793,507, now abandoned the initial epoxy resin, that is employed in the graft polymer production process, may be terminated to eliminate part or all of the terminal epoxy groups, so as to reduce or eliminate the possibility of ester grafting.
In these prior applications, the initial epoxy resin is modified to make it dispersible in water. This is accomplished by incorporating ionizable groups in the epoxy resin molecules. In Ser. No. 793,507, now abandoned it is disclosed that these ionizable groups may be acid-functional or base-functional, and that base functionality can be obtained by the introduction of amino nitrogen into the epoxy resin molecule by the reaction of a primary or secondary amine with the terminal epoxy groups; see Ex. 6 of Ser. No. 793,507 now abandoned.
The ionized reaction products of these prior applications appear to have remarkable properties. They are believed to serve as the means by which the ungrafted epoxy resin component is kept in stable suspension.
While resinous coating compositions of the kinds disclosed in these prior applications have excellent functional characteristics and other highly desirable properties, their high content of epoxy resin increases their cost. In addition, these resinous coating compositions contain liquid organic solvents, which are used to facilitate handling during the manufacturing process, and to improve application properties such as sprayability.
In another related patent application, Ser. No. 029,106 filed Apr. 11, 1979, now U.S. Pat. No. 4,285,847 the disclosure which is also incorporated herein be reference, the aqueous dispersions described in these prior applications are used as vehicles in which to conduct in situ vinyl polymerizations. One preferred vinyl monomer that can be used for this purpose is styrene, although mixtures of monomers may be employed. The in situ vinyl polymerization step permits the addition of more water, and thus serves, while increasing the content of solids in the composition, to reduce the proportion of liquid organic solvent in the overall composition.